Today’s a prime day. Whether you write the date in American (MMDDYY), European (DDMMYY), or ISO (YYYYMMDD) format, you get a prime. That is, 112913 and 291113 and 20131129 are all prime numbers.
We’ll call a date an American prime date if MMDDYY is prime, a European prime date if DDMMYY is prime, and an ISO prime date if YYYYMMDD is prime. (Single-digit days and months are padded with a zero.) If a date is prime by all three criteria, we’ll call it an international prime date. Today is an international prime date, and there won’t be another one until August 11, 2019.
If a date is an American prime date and a European prime date, we’ll call it a transatlantic prime date. After today, the next transatlantic prime dates are December 4 and 13 this year. There will be no transatlantic prime dates in 2014, 2015, and 2016 since these dates correspond to numbers that are divisible by either 2 or 5. The first transatlantic prime date of 2017 will be January 16.